Until recently it had been thought that the adult heart could not generate new myocytes. Although this dogma is changing, we still understand very little about the endogenous cardiac repair process. Since cardiac injury causes cell death, repair should involve replacing these cells. Our preliminary experiments have shown that a 7-day infusion of isoproterenol (ISO) induces myocyte loss, replacement fibrosis and depressed cardiac pump function. Several weeks after ISO infusion, we have observed improved cardiac function, suggestive of endogenous repair. Specific Aim 1: To determine if catecholamine-induced myocyte death is associated with an increase in endogenous stem cell derived new myocyte formation, a) Myocyte death will be induced by a continuous 7-day infusion of isoproterenol. b) New myocyte formation will be determined by measuring BrdU labeled cardiac myocyte nuclei, c) We will correlate injury and endogenous recovery with myocyte death, fibrosis, and new myocyte formation. Specific Aim 2: a) To determine if either bone marrow cells (BMCs) or cardiac stem cells (CSCs) can improve the function of the injured feline heart. In vivo cardiac function and wall thickness will be assessed using echocardiography and invasive hemodynamic measurements at baseline, following cardiac injury, and after injection of human bone marrow and/or human resident cardiac stem cells, b) To determine if injected human derived BMCs and human resident CSCs can differentiate into cardiac myocytes and other cardiac cell types. These two cell types will be studied individually (in separate animals) and in a unique side-by-side comparison by injecting both cell types into the same damaged heart. BMCs and CSCs will be loaded with negatively charged, distinctly colored, quantum dots (QDots) for tracking of each cell type. Perfusion fixed and sectioned hearts will be used to determine the fate of injected QDot loaded BMCs and CSCs. Specific Aim 3: To determine if human BMC and CSC-derived myocytes differentiate into fully functional ventricular myocytes when injected into the damaged feline heart. Bone marrow and CSC-derived newly formed myocytes will be digested from damaged hearts, identified by quantum dot fluorescence, and then studied. Electrophysiology (AP characteristics and T and L type Ca2+ currents), Ca2+ transients, contractions, and cell size and nucleation will be assessed. These characteristics will be compared to those of adult cardiac myocytes from the same hearts. According to the American Heart Association, about 1 in 3 American adults have one or more types of cardiovascular disease (CVD). Current therapeutics are aimed at decreasing the severity of CVD, but prognosis remains poor. Cardiovascular cell therapy has the potential to greatly improve this outcome.